The present invention relates to an electrophotosensitive material containing a quinone derivative having an excellent electric charge transferability, which is used in image forming apparatuses such as electrostatic copying machine, facsimile and laser beam printer.
In the image forming apparatuses, various photosensitive materials having the sensitivity within a wavelength range of a light source used in the apparatus are used. One of the photosensitive materials is an inorganic photosensitive material using an inorganic material such as serene in a photosensitive layer and another one is an organic photosensitive material (also referred to as an organic photoconductor, OPC) using an organic material in a photosensitive layer. Recently, the organic photosensitive material among these photosensitive materials has widely been studied because it is easily produced as compared with a conventional inorganic photosensitive material and that it has advantages such as wide range of choice of photosensitive materials such as electric charge generating material, electric charge transferring material and binder resin, and high functional design freedom.
The organic photosensitive material includes a so-called multi-layer type photosensitive material having a laminated structure of an electric charge generating layer containing an electric charge generating material and an electric charge transferring layer containing an electric charge transferring material, and a so-called single-layer type photosensitive material wherein an electric charge generating material and an electric charge transferring material are dispersed in the same photosensitive layer. Among these photosensitive materials, the multi-layer type photosensitive material is exclusively put into practical use and, furthermore, a multi-layer type photosensitive material comprising an electric charge transferring layer having a film thickness larger than that of the electric charge generating layer at an outermost layer is used more generally in view of the mechanical strength.
It is required that the electric charge transferring material used in these organic photosensitive materials has high carrier mobility. However, since almost all of electric charge photosensitive materials having high carrier mobility have hole transferability, the multi-layer type photosensitive material comprising the electric charge transferring layer at the outermost layer becomes a negatively charging type one inevitably.
However, this negatively charging type organic photosensitive material must be charged by negative corona charge accompanied with the generation of a large amount of ozone, thereby to cause problems such as influence on the environment and deterioration of the photosensitive material itself.
To solve the problems described above, it has been studied to use an electron transferring material as the electric charge transferring material, thus suggesting, as the electron transferring material, a diphenoquinone derivative represented by the general formula (EA1): 
wherein RA, RB, RC and RD are the same or different and each represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, an aryl group having 6 to 12 carbon atoms, an aralkyl group having 6 to 12 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, an alkoxy group having 1 to 4 carbon atoms, or an amino group which optionally has an alkyl group having 1 to 4 carbon atoms, or a benzoquinone derivative represented by the general formula (EA2): 
wherein RE, RF, RG and RH are the same or different and each represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, an aryl group having 6 to 12 carbon atoms, an aralkyl group having 6 to 12 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, an alkoxy group having 1 to 4 carbon atoms, or an amino group which optionally has an alkyl group having 1 to 4 carbon atoms.
Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Hei) No. 6-110227 suggests to use, as the electron transferring material, a naphthoquinone derivative represented by the general formula (EA3): 
wherein RJ represents a hydrogen atom, a substituted or non-substituted alkyl group, a substituted or non-substituted phenyl group, a halogen atom, an alkoxycarbonyl group, a N-alkylcarbamoyl group, a cyano group or a nitro group, and xe2x80x98axe2x80x99 represents an integer of 1 to 3.
However, injection of electrons into the electron transferring material from the electric charge generating material was insufficient because of poor matching of conventional electron transferring materials such as diphenoquinone derivative (EA1), benzoquinone derivative (EA2) and naphthoquinone derivative (EA3) with the electric charge generating material. Since such an electron transferring material has low compatibility with a binder resin and is not uniformly dispersed in a photosensitive layer, the hopping distance of electrons becomes longer and electrons are less likely to move at low electric field.
Accordingly, as is apparent from electrical characteristics test described in Examples described hereinafter, the above-described conventional photosensitive material containing an electron transferring material had problems such as high residual potential and poor sensitivity.
The single-layer photosensitive material has advantages that one photosensitive material can be used in both of positively and negatively charging type apparatuses by using electron and hole transferring materials in combination. However, there arise problems that, when using diphenoquinone derivative (EA1), benzoquinone derivative (EA2) and naphthoquinone derivative (EA3) described above as the electron transferring material, a charge transfer complex is formed by an interaction between the electron and hole transferring materials, thereby inhibiting transfer of electrons and holes.
Thus, an object of the present invention is to solve the technical problems described above and to provide an electrophotosensitive material whose sensitivity has been improved as compared with a conventional one.
While studying intensively to solve the problems described above, the present inventors have paid attention to the fact that (a) a planar structure molecule having an acceptable functional group is generally suited for use as an electron transferring material, however, (b) such a molecule is of no practical use because of poor compatibility with a binder resin, and they have studied furthermore to enhance the electron acceptability and electron transferability by further introducing an acceptable functional group into a diphenoquinone derivative as a conventional typical electron transferring material, and to improve the solubility of the compound and compatibility with the binder resin.
As a result, they have found a new fact that:
(A) a furan ring or thiophene ring is introduced in the direction of a quinone functional group of a diphenoquinone derivative to extend a xcfx80 electron conjugated system in said direction and a predetermined substituent is introduced into the quinone functional group, or
(B) a naphthalene ring or anthracene ring is introduced in the direction of a quinone functional group of a diphenoquinone derivative to extend a xcfx80 electron conjugated system in said direction and the direction which intersects perpendicularly to said direction and a predetermined substituent is introduced into the quinone functional group, thereby making it possible to obtain a compound having high electron transferability and improved solubility and compatibility with the binder resin, and that an electrophotosensitive material which has low residual potential as compared with a conventional one and high sensitivity can be obtained by incorporating such a compound as the electron transferring material. Thus, the present invention has been completed.
First electrophotosensitive material according to the present invention comprises a conductive substrate and a photosensitive layer formed on the conductive substrate, the photosensitive layer containing a quinone derivative represented by the general formula (1): 
wherein A represents an oxygen atom or a sulfur atom, and R1, R2, R3 and R4 are the same or different and each represents an alkyl group having 1 to 4 carbon atoms, or an aryl group having 6 to 12 carbon atoms, which optionally has an alkyl group having 1 to 4 carbon atoms.
Second electrophotosensitive material according to the present invention comprises a conductive substrate and a photosensitive layer formed on the conductive substrate, the photosensitive layer containing a quinone derivative represented by the general formula (2): 
wherein R5, R6, R7, R8 and R9 are the same or different and each represents an alkyl group having 1 to 4 carbon atoms, or an aryl group having 6 to 12 carbon atoms, which optionally has an alkyl group having 1 to 4 carbon atoms, and m represents an integer of 0 to 4.
Furthermore, third electrophotosensitive material according to the present invention comprises a conductive substrate and a photosensitive layer formed on the conductive substrate, the photosensitive layer containing a quinone derivative represented by the general formula (3): 
wherein R10, R11, R12, R13 and R14 are the same or different and each represents an alkyl group having 1 to 4 carbon atoms, or an aryl group having 6 to 12 carbon atoms, which optionally has an alkyl group having 1 to 4 carbon atoms, and n represents an integer of 0 to 4.
The quinone derivative (terphenoquinone derivative) represented by the general formula (1) used as the electron transferring material in the electrophotosensitive material of the present invention has a very wide xcfx80 electron conjugated plane because the xcfx80 electron conjugated system is extended in the direction of the quinone functional group as compared with the diphenoquinone derivative as a conventional electron transferring material and the furan and thiophene rings having high electron acceptability are introduced into the center of a molecular skeleton, as described above. Therefore, the quinone derivative (1) has remarkably excellent electron acceptability in the whole molecule.
Furthermore, since the furan and thiophene rings as the five-membered ring exist in the center of the molecular skeleton, the molecule is bent at the portion of the five-membered ring while maintaining the xcfx80 electron conjugated plane, thereby lowering symmetry of the whole molecule as compared with the diphenoquinone derivative. Therefore, the quinone derivative (1) has high solubility and good compatibility with the binder resin because an alkyl group and a phenyl group are substituted at both ends of the molecule, thereby making it possible to uniformly disperse in the photosensitive layer.
The quinone derivatives represented by the general formulas (2) and (3) used as the electron transferring material in the electrophotosensitive material of the present invention also have a very wide xcfx80 electron conjugated plane similar to the quinone derivative of the general formula (1) because the xcfx80 electron conjugated system is extended in the direction of the quinone functional group as compared with the diphenoquinone derivative as a conventional electron transferring material and also the xcfx80 electron conjugated system is extended on almost the same plane as that of the benzene ring constituting the quinone functional group in the direction which intersects perpendicularly to said direction in the center of a molecular skeleton, as described above. Therefore, the quinone derivatives (2) and (3 ) have remarkably excellent electron acceptability in the whole molecule.
Furthermore, the quinone derivatives (1) to (3) are superior in matching with the electric charge generating material and injection of electrons from the electric charge generating material is smoothly conducted. Accordingly, the quinone derivatives (1) to (3) exhibit excellent electric charge transferability even at low electric field and are suited for use as the electron transferring material in the electrophotosensitive material. Moreover, since the quinone derivatives (1) to (3) do not form a charge transfer complex with the hole transferring material, they are used particularly preferably in the single-layer type photosensitive layer using the electron transferring material in combination with the hole transferring material.
The photosensitive layer containing the quinone derivatives (1) to (3) is superior in electron transferability at low electric field and is less likely to cause recombination of electrons and holes in the photosensitive layer, whereby apparent electric charge generation efficiency approaches an actual value. As a result, the sensitivity of the photosensitive material having such a photosensitive layer is improved. The residual potential of the photosensitive material is also lowered, thereby improving the stability and durability on repeated exposure.
Since the quinone derivatives (1) to (3) do not form a charge transfer complex with the hole transferring material as described above, a photosensitive material having higher sensitivity can be obtained when using them in a single-layer type photosensitive material containing the electron transferring material and hole transferring material in the same photosensitive layer.
In the electrophotosensitive material of the present invention, when the photosensitive layer contains a compound having a redox potential of xe2x88x920.8 to xe2x88x921.4as the electron acceptor, together with the quinone derivatives (1) to (3) (electron transferring material), the sensitivity of the photosensitive material is further improved.
The reason is assumed as follows. That is, the electron acceptor extracts electrons from the electric charge generating material and transfers the electrons to the quinone derivatives (1) to (3) as the electron transferring material, whereby the electrons are injected into the quinone derivatives (1) to (3) from the electric charge generating material more smoothly.
Considering the combination with the quinone derivatives (1) to (3), it is preferable to use those having a redox potential within the above range among the diphenoquinone derivative represented by the general formula (EAl) and benzophenone derivative represented by the general formula (EA2).